Information recognition systems are well known in the art. Extensive use of bar code symbols for encoding various items has gained widespread acceptance. Bar code readers detect the information of the bar code from the difference in reflection between dark bars and light spaces or white bars. In a typical bar code reader configuration, an illumination light is supplied to a bar code and is reflected by the bar code according to the dark and light bars. The reflected light is converged and received by a lens and a light responsive image sensor. Since the reflected light has a light density corresponding to the light reflectivity of the bar code, the bar code is converted by the image sensor into an image signal which has a signal level corresponding to the density of the reflected light. This image signal is then processed electronically for recognizing the bar code symbol.
Since bar code readers detect the information of the bar code from the difference in reflection between dark and light bars, the amplitude of the output signal from the reading sensor is small when the reflection factor is low and when reading a bar code printed on a high reflection factor label, the output signal exhibits a large amplitude. Therefore, the output signal level changes due to the difference in reflection factor of the bar code labels. The reflection factor is influenced by the amount of light impinging upon the bar code. Problems encountered with bar code readers using illumination light sources include a nonuniform illumination of the bar codes, an illumination level that varies with the distance between the bar codes and the reader and an illumination level that varies with the angular orientation of the reader to the bar codes. The use of an illumination source having an increased intensity lessens some of these problems. However, increased light intensity results in an increase in the space occupied by a light source, increased power consumption and increased heat generation. Additionally, the intensity of the reflected light changes when the intensity of the incident light changes. Since external light is also incident on the label, the intensity of the reflected light is affected even if the intensity of the light from a built-in light source is kept constant.
A need has thus arisen for a bar code reader which utilizes a charge transfer device image sensor which eliminates the problems associated with a built-in illumination light source by solely utilizing ambient light as the bar code illumination source. The use of ambient light uniformly illuminates the bar codes and renders the intensity of the reflected light independent of the separation between the bar code reader and bar codes. Ambient light as an illumination source also renders the bar code reader less sensitive to the angular position of the reader to the bar code, such that the intensity of the reflected signal is independent of the angular position. The use of ambient light further allows for a large range of light intensity which must be accommodated by a charge transfer device image sensor. Such a bar code reader must have a large depth of field while being capable of reading a full range of bar code densities.